In recent years, there has been an increasing demand for a high-performance lithium secondary battery for use, for example, in portable information terminals, portable electronic devices, domestic small electronic energy storage devices, two-wheel motor bicycles using a motor as a power source, electric automobiles, and hybrid electric automobiles. With such a trend, further improvement in safety and performance of secondary batteries has been desired.
Conventional electrolytes which exhibit high lithium ion conductivity at room temperature are mostly liquids. For example, organic liquid electrolytes are known to be a material which exhibits high lithium ion conductivity at room temperature.
Conventional organic liquid electrolytes are flammable since they contain an organic solvent. Therefore, actual use of an ion conductive material which contains an organic solvent as a battery electrolyte involves concern for liquid leakage or risk of ignition.
Furthermore, due to its liquidity, not only lithium ions but also counter anions are also conducted in such an electrolyte. Therefore, the lithium ion transport value thereof is not 1. In addition, exposure of such a battery to high temperatures (to 300° C.) results in decomposition and vaporization of the electrolyte, causing bursting or other problems of a battery. Due to these disadvantages, the organic liquid electrolyte has only a limited application range.
To ensure safety, studies have been made on the use of an inorganic solid electrolyte instead of an organic solvent electrolyte as the electrolyte for secondary batteries. The inorganic solid electrolyte is inflammable or hardly flammable by nature, and is a safe material as compared to an electrolyte generally used. Under such circumstances, development of all-solid lithium batteries having a high degree of safety has been desired.
To meet such a demand, various studies have been made on sulfide-based solid electrolytes. As lithium ion conductive solid electrolytes exhibiting a high ionic conductivity, sulfide glass having an ionic conductivity of 10−3 S/cm has been found in the 1980s, examples of which include LiI—Li2S—P2S5, LiI—Li2S—B2S3 and LiI—Li2S—SiS2 (see Non-Patent Document Nos. 1 and 2, for example). These electrolytes are free from problems such as ignition and bursting. However, due to the low glass transition temperature or phase transition temperature, these electrolytes encounter a problem in which the performance thereof deteriorates when exposed to temperatures around 280° C.
Characteristic features of all-solid lithium batteries using a solid electrolyte reside in that they are capable of being operated at significantly high temperatures as compared with lithium batteries using an organic liquid electrolyte, and they are possibly resistant to solder reflowing. For the protection of environment, lead-free solders have come to be used instead of lead solders. While lead soldering is conducted at 230° C. to 240° C., lead-free soldering is conducted at 260° C. to 290° C. There is a tendency that reflow soldering temperature has been on the increase. Thus, solid electrolytes which are more improved in heat resistance have been required.
Non-Patent Document 1: H. Wada, Mater. Res, Bul 18 (1983) 189
Non-Patent Document 2: Ren't Mercier, solid state Ionics 5 (1981) 663-666
The invention has been made in view of the above-mentioned problems. An object of the invention is to provide a solid electrolyte having a high lithium ion conductivity and heat resistance, and to provide an all-solid lithium solid battery improved in heat resistance.